Heating coils employed in electrical furnaces operative at exceedingly high temperatures are typically supported by ceramic cores such as grooved plates or cylinders wherein the heater is supported and often confined throughout its entire length by the ceramic structure. The weight of the ceramic support structure constitutes a major percentage of the overall heater assembly mass by reason of the amount of ceramic necessary for support of the heating element and the inherent density of the ceramic material. As a result of the relatively massive amount of ceramic material present in a heater assembly of conventional construction, the heater exhibits a high thermal inertia which limits the rapidity with which a change of temperature can be accomplished. The response of such conventional heaters to temperature control is thereby limited by the relatively slow thermal response of the heater structure.
The function of the ceramic core in each of these prior heaters is to support and contain the electrical heating element. The core may be composed of a cylindrical rod or a circular or rectangular plate having a plurality of longitudinal re-entrant slots or grooves formed in the peripheral surface thereof and rubbing the length of said surface. These grooves, due to the limitations imposed by the ceramic material, are necessarily of small diameter and will expose at the maximum one fifth the surface area of the electrical heating element itself. The ceramic core therefore effectively shades at least 80% of the direct radiation emitted by the coil to the product, thus providing a low standard of emissivity. This low emissivity in turn promotes a substantial differential in temperature between the product and the heating element, causing efficiency and shorter heater life.
Another well known form of heating coil employs three-eighths inch relatively heavy heating rod, typically three/eighths in diameter, wound in a helical configuration. Ceramic spacers are interposed between turns of the helix to maintain spacing of the heater turns. This type of heater construction depends on the radial arch of the heater rod for support, and the rod has to support itself as well as the interposed ceramic spacers. The additional weight presented by the ceramic spacers contributes to sagging of the heating rod at high temperatures and shading of a significant portion of the heating surface.
Examples of prior devices are shown in U.S. Pat. Nos. 2,870,308; 3,651,304; 3,673,387; 3,783,238 and 3,798,417. A high temperature heater which overcomes the deficiencies of the prior art is the subject of copending application Ser. No. 622231, filed of even date herewith, entitled ELECTRICAL RESISTANCE FURNACE HEATER and assigned to the same assignee as the present invention.